Starch is a complex carbohydrate made up of two primary components: amylose and amylopectin. Starch comes from a variety of sources, and the relative amounts of amylose and amylopectin in the starch can vary depending upon the source. Sources of starch include roots/tubers (potato, tapioca, and arrowroot) and cereal (wheat, corn, rice).
The properties of different types of starch depend at least in part on the relative amounts of amylose and amylopectin. Table 1 summarizes the amounts of amylose and amylopectin found in various types of starch.
TABLE 1STARCH% AMYLOSE% AMYLOPECTINTapioca1783Potato2080Wheat25-2674-75Corn24-2872-76Waxy corn 0100 Rice2278
Starches with a higher amount of amylopectin, such as tapioca and potato starches, tend to be good thickening agents and form soft gels, as described below. Starches with greater amounts of amylose, such as corn starch, tend to form stronger gels. Mixtures of root and cereal starches are often used, such as mixtures of potato and corn starches.
Starch is formed as granules in the cytoplasm of plant cells. Amylopectin forms in concentric spheres with amylose dispersed in between. The granules are held together by hydrogen bonds, and swell when heated in aqueous liquids. When the granules swell, water can migrate into the spheres of the granules. The reduced free water left in the liquid causes a change in the viscosity, thickening the liquid. This process is sometimes referred to as gelatinization, and is what gives structure to baked products and thickens sauces, soups and dressings. The relative amounts of starch needed to thicken a starch-based composition can be seen in Table 2.
TABLE 2Comparative AmountNeeded to AchieveDesignated Viscosity of HotType of Starch(approx 95° C.) Starch PastePotato1.96Waxy corn2.98Waxy rice3.13Waxy sorghum3.42Tapioca3.54Cross-linked4.15waxy cornArrowroot4.37Sorghum4.66Corn4.90Waxy rice flour5.48Rice5.49Rice flour5.57Wheat6.44Wheat flour9.27Adapted from Osman, E. M. and Mootse, G. 1958, Food Res. 23, 554.
The temperature to which the mixture must be heated to effect gelatinization of the starch is sometimes referred to as the gelatinization temperature, and will depend upon the type of starch. Other substances in the mixture, such as sugar, can raise or lower the gelatinization temperature.
As the thickened starch mixture cools, a gel is formed as hydrogen bonds hold the swelled granules in place. The amylose that was released from the granule as the water was absorbed also participates in the hydrogen bonding and gel formation. This process is sometimes referred to as gelation.
There are problems that can arise with starch-thickened products. If too many of the granules are damaged due to extreme heat or agitation, loss of viscosity or phase separation of the thickened mixture can result (scrambling). Starch mixtures are typically heated slowly, allowing a gradual swelling of the granules. The mixture is typically stirred, but vigorous agitation is avoided, so as to avoid mechanical damage to the granules.
Thus, classically prepared starch-thickened products, such as custards, require a careful cooking process. Heated milk or cream is added to an egg, sugar and starch mixture. This step is referred to as tempering. The tempered mixture is then carefully cooked to a simmer until the gelatinization temperature is reached, resulting in gelatinization (thickening) of the starch. Continual stirring and observation of the mixture is required, to avoid scorch or curdling. Fifteen to twenty minutes of labor-intensive cooking may be necessary to thicken the mixture.
The amount of heat must be carefully controlled and limited. If excessive heat is used to speed up the process, a scorched and/or scrambled product may be produced, resulting in wasted ingredients and time.
The process of thickening starch-based products is therefore time consuming, requires skill and experience, and is labor-intensive. The need exists, therefore, for a method of preparing starch-thickened compositions that is less labor-intensive and reduces the possibility of wasted ingredients and time.